A 400 million-year-old fossil fish with a reproductive organ resembling a penis has been identified by Australian scientists. This is the earliest known structure used for sexual reproduction as we know it. The bone attached to the pelvis is called a clasper, and was used to penetrate a female during mating. The fish was a member of the extinct class of armored fish called placoderms.

Study author and palaeontologist Dr Kate Trinajstic, of Curtin University in Perth, says the clasper was discovered in a fish specimen uncovered in the Gogo region of Western Australia in 2001.

She says the team originally discounted the bone as the reproductive organ because they thought it was part of the pelvic gurdle.

On closer inspection, Trinajstic says they realised it was a sexual organ.

"We were surprised because it's so big," she says. "We were expecting something smaller."